Sled dogs at the at the ceremonial start of the Iditarod (from Mitch Seavey's 2010 team)

Frank Kovalchek (Creative Commons Attribution 2.0 Generic license)

“It is so instinctual to be doing what these dogs are doing...” Iditarod contestant and avid musher Mike Santos believes, “...That it really requires very little training.” Dogs love to run. Still, a musher’s challenges are daunting. Alaskan weather is fierce and unpredictable; handling logistics, supplies, the vagaries of trail conditions, and– perhaps most of all– knowing the capabilities of yourself and your team are vital for every racer. Santos chooses his sled dog team carefully. “Just like people, they are all suited to different temperatures, different trail lengths, team size,” &c. Mike Santos owns Wolf’s Den Kennel in Cantwell, Alaska, which houses about 65 dogs.

Calorie counting

Every sled dog racing the Iditarod Trail Sled Dog Race consumes roughly 12,000 calories daily, the equivalent of 24 McDonald’s Big Macs. Yet they weigh only about 40-60 pounds [18-27 kilograms]. That makes sled dogs powerful calorie burners; in contrast, human athletes struggle to put away more than 5,000 calories in one day despite weighing in at roughly three times as much as a racing sled dog. Long-distance canine racers typically burn 240 calories per pound of weight every day, while a high-performance human athlete, a Tour de France cyclist, would typically burn only 100 calories per pound of weight daily.

The Iditarod race

Iditarod Trail Sled Dog Race, the world's longest sled race, takes teams of canine athletes and human mushers along a grueling 1,100 mile [1770 kilometer] long trail from Anchorage west to Nome, Alaska. Wild weather is the norm. and every team can anticipate blizzards, ice and fog, screaming winds strong enough to tip sleds and temperatures as low as -40°F [-40°C].

Teams are composed of 12 to 16 dogs, and there must be at least 6 dogs pulling the racer’s sled at the finish line.

Mushers provide careful care for their dogs, dressing paws and groins to protect against ice as needed, keeping the dogs hydrated, and feeding them a mixture of meat, fish, and commercial dog food (typically 60% fats, 40% carbohydrates and proteins). Checkpoints are set up along the way where mushers check in, dogs rest, and veterinarians sign off on the health of every animal.

Alaskan huskies and other long-distance racing dogs are mighty endurance athletes. These sled dogs can run an average of eight miles per hour, traversing over 100 miles per day.

The secret to the dogs’ mighty endurance, and their calorie-burning capabilities, lies in their powerful and unusual metabolism.

Normal Metabolism

When we say metabolism, we reference bodily processes that convert or use energy. Think digestion, muscle use, blood circulation, breathing, and the functioning of nerve networks and the brain, and much more. Metabolism includes catabolism that breaks down i.e. nutrients into energy, and anabolism, which uses that energy to construct things we need.

We gain energy from catabolism. Proteins are broken down into amino acids and can be converted into other compounds– used to make glucose, for instance. Glucose is carried by the blood stream. It is also stored in muscles and in the liver as glycogen, which is a complex carbohydrate. Any extra energy that humans get from metabolism is stored either as glycogen or as fat.

Glycogen is useful; it is very important to the function of muscles. Muscles use glycogen as fuel to power physical activity.

Strenuous exercise

Heavy exercise prompts muscles to use glycogen. As human endurance athletes exercise they burn calories, deplete their glycogen and thus their muscular energy reserves, and stress their bodies. Marathon runners and athletes undergoing intensive training diminish stores of glycogen and protein, build up lactic acid, and eventually become fatigued at which point they must rest and recover until their stores are replenished.

Dogs normally metabolize energy as explained above. Long-distance endurance runs, however, cause sled dogs’ metabolism to radically alter. It has been compared to throwing a metabolic switch. Veterinarian Michael Davis, Oklahoma State University Center for Veterinary Health Sciences, has been investigating the phenomenon since 2001.

“Before the race, the dogs’ metabolic makeup is similar to humans. Then suddenly they throw a switch — we don’t know what it is yet — that reverses all of that,” Davis explained. “Dogs will go from using their reserves to not.” In 24 to 48 hours, endurance-conditioned sled dogs “Change their metabolism so they don’t use up their reserves anymore.” Within four days the dogs can “Go back to the same type of metabolic baseline you see in resting subjects. But it’s while they are running 100 miles a day.”

Despite consecutive days of strenuous exercise, the dogs don’t become fatigued like human endurance athletes do. Instead, they rapidly adjust to the demands of running the race. This physiological capability makes sled dogs the perfect athlete for the Iditarod.

Davis said, you can “Take dogs out and you run them 100 miles per day today and tomorrow and the next day, and they come back, sleep, eat, do it again without having any outward sign of it mattering.” In fact when the dogs run in more than one strenuous race in the season “They don’t just continue to perform, they perform a lot better.” Davis pointed out “They can do it indefinitely, as long as you have trail and they’ve got food. They get tired, but they don’t fatigue in the biochemical sense.” They are essentially fatigue proof.

Could humans do it too?

Dr. Davis has been analyzing blood and muscle samples from long-distance running canines in Alaska to try to discover what flips that metabolic switch. Humans could benefit deeply from the discovery. Imagine: greater endurance for athletes, avoiding fatigue for i.e. armed forces, and perhaps even benefits for diabetics and another tool in the fight against obesity. Davis speaks of the dogs: “They have a hidden strategy that they can turn on,” and “We are confident that humans have the capacity for that strategy. We have to figure out how dogs are turning it on,” in order “To turn it on in humans.”

Getting oxygen moving

Long-distance canine athletes have powerful lungs and circulatory systems which are capable of delivering plenty of oxygen to their muscles. The ratio of the volume of oxygen delivered to the body’s weight per minute is called aerobic capacity. An elite human athlete’s aerobic capacity is something like 60-80 ml/kg/min Vo2max. An untrained sled dog’s is 175 ml/kg/min VO2max. A trained and racing sled dog’s average aerobic capacity is roughly 300 ml/kg/min VO2max. That great ability to move oxygen to the muscles that need it lends itself well to a sled dog’s repertoire.

What’s more, racing sled dogs are great at moving energy swiftly into their muscle cells.

Converting fat to fuel

Glycogen, the complex carbohydrate compound which stores energy to supply to muscles, is utilized by the sled dogs at first. After the metabolic switch is flipped in racing dogs the animals use a glycogen-sparing metabolism instead. Their glycogen stores replenish as opposed to becoming depleted. Instead of converting proteins or fats into glycogen, they seem to be pulling fat from their bloodstream into their cells and utilizing it for energy. Davis’ findings suggest that the canines are directly burning fat.

It might be made possible by the dogs’ surplus of mitochondria: cellular power plants. Dogs have roughly 70% more mitochondria per cell than humans have. Davis speculates that the canines are extracting fat from the bloodstream and somehow moving it into the mitochondria of their muscle cells. We’re not certain how that transportation takes place, but the hormone insulin seems to play an important role. With their high-fat racing diet, the use of fat for energy, and their numerous mitochondria, dogs can obtain and burn fuel very efficiently.

“The faster you can get stuff into a muscle cell, the faster you can use it,” Davis said. When the switch is thrown, “They may get better at pulling fat out of the bloodstream on the fly.”

Transporters are proteins that move fatty acids, glucose and complex carbohydrates from the bloodstream across cell membranes and into the mitochondria power plants. Davis is hard at work decoding the mystery of the canines’ transporters; normally dogs use transporters similar to the ones humans use, but not during endurance races like the Iditarod. “Something is transporting the fat into dog muscle,” he said, “But it isn’t the transporter that we [humans] use.”

2014 Race

Congratulations to Dallas Seavey, winner of the 2014 Iditarod Trail Sled Dog Race, and to all contestants entered. As always, navigating trail conditions involved much more than “Gee” (left turn) and “Haw” (right turn). In Alaska, “Changes in weather can occur almost instantaneously because of geography or just because of wind change,” musher Mike Santos noted. This year proved challenging because of unusual warmth and stretches of bare ground, but that’s not to say the mushers didn’t run into blizzards and icy tundra. “The Iditarod is always challenging,” Santos said, and “I think you would be very foolish to sign up for the Iditarod and not expect to have a pretty big challenge ahead of you.” Of course, it’s a challenge that he and other mushers relish.

Frontier Scientists:presenting scientific discovery in the Arctic and beyond